Kaptai Lake is the largest artificial reservoir in South-East Asia and very rich in inland open water capture fisheries of Bangladesh. A study was conducted in three different communities viz. Natun Jele Para (mostly Hindus and entirely depends on fishing at lake), Adar Pahar (mostly Muslim and involved in various occupation) and Mitingachori (mostly ethnic group called Chakma, not entirely dependent on the lake fishery) to understand the historic changes of lake fish biodiversity. PRA tools, well-being ranking of the community people, were used to collect data. It was obtained mixed livelihood profiles in three communities but almost similar views on about the biodiversity changes of fishes in lake. About 44 fish species were recorded and ranked in terms of their availability within the last few decades. Indigenous and coastal fish species were available before the embankment (1960). Then some exotic carp species like Chinese carp, Big Head Carp etc. were dominated in the catch composition but gradually some Small Indigenous Species (SIS) was remaining in the top of the fish catch chart. Now, both Ganges River Sprat (Corica soborna) and Indian River Shad (Gudusia chapra) comprises about 60% of the total catch with an equal share of each. In 2013-14, total fish production was 8,813 MT (metric ton) where the overall productions of Indian Major Carps (IMC’s) fish like Labeo rohita, Gibelion catla, Cirrhinus cirrhosus were only 38.44 MT. On the other hand C. soborna and G. chapra production were 2,581.53 MT and 2,640.45 MT respectively. Recent landing data of Bangladesh Fisheries Development Corporation (BFDC) were reviewed and it also supports the dramatic increase trend of SIS. The indiscriminate fishing, sedimentation in the lake, water pollution, changing the rainfall hinder the breeding performances of IMC’s, and use of Keski Net damage other ichthyoplankton especially IMC’s fries.